1. Technical Field
This invention relates to noise, and more particularly, to a system that estimates noise.
2. Related Art
Some communication devices receive and transfer speech. Speech signals may pass from one system to another through a communication medium. In some systems, speech clarity depends on the level of noise that accompanies the signal. These systems may estimate noise by measuring noise levels at specific times. Poor performance in some systems may be caused by the time varying characteristics of noise that sometimes masks speech.
In other systems, noise is monitored during pauses in speech. When a pause occurs, an average noise condition is recorded. Through spectral subtraction an average noise level is removed to improve the perceived quality of the signal. In vehicles and other dynamic-noise environments, systems may not identify noise, especially noise that occurs during speech. A sudden change in a noise level that occurs, for example, when a window opens, a defrosting system turns on, or when a road transitions from asphalt to concrete may not be identified, especially if those changes occur when someone is speaking.
Some alternative systems track minimum noise thresholds. When no signal content is detected, noise is monitored and a minimum noise threshold is adjusted. If sudden changes in noise levels occur, some systems adjust the minimum noise threshold to match the change in noise levels. These systems may offer improved performance in high signal to noise conditions but suffer when the systems attempt to remove speech that may occur, for example, in echo cancellation. In some systems, echoes are replaced with comfort noise that tracks the minimum noise thresholds. In a worst case scenario, the perceived quality of speech may drop as the background noise tracks the fluctuating noise thresholds. There is a need for a system that improves noise estimates.